Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1999-01-15
2001-07-24
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C250S492200, C250S492220, C356S399000, C356S400000, C356S401000
Reexamination Certificate
active
06266130
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a position detecting method and a position detecting system for optically measuring, at a high precision, the position of an object to be detected and for aligning the object with respect to a predetermined position. For example, the invention is suitably usable in a semiconductor device manufacturing projection exposure apparatus, called a stepper, for sequentially projecting and printing an image of a circuit pattern formed on the surface of a reticle onto pattern regions upon a wafer through a projection lens system, to produce semiconductor devices.
In a projection exposure apparatus called a stepper, for the manufacture of semiconductor devices, a reduction projection optical system is used to sequentially project an image of a circuit pattern formed on a reticle onto pattern regions upon a wafer in a reduced scale, by which the circuit pattern is transferred to these regions. During This pattern transfer process, in the stepper, each pattern region on the wafer is brought into accurate alignment with respect to the reticle and, thereafter, the projection exposure operation is performed.
A wafer has plural registration or alignment marks formed thereon for the registration. The stepper is provided with an alignment microscope for detecting alignment marks so as to obtain positional information related to pattern regions on the wafer. On the basis of the positional information about the alignment marks thus obtained through the alignment microscope, the position of the wafer with respect to the reticle is precisely detected in the stepper, and thus, each pattern region of the wafer can be accurately brought into alignment with the reticle.
As regards an alignment method based on such an alignment microscope, there are a TTL alignment method (TTL system) and an off-axis alignment method. In the TTL alignment method, observation is made through a reduction projection optical system and, therefore, it is called a TTL (Through The Lens) system. The alignment microscope serves to observe alignment marks on the after surface by use of illumination light having a wavelength different from exposure light and with respect to which wavelength a resist applied to the wafer is non-sensitive. On the basis of the result of observation, the wafer alignment is performed.
On the other hand, the off-axis alignment method is a detecting method without the use of a projection optical system. Positional information is detected by use of a microscope (off-axis microscope) which is disposed at a position separate from the wafer exposure position. Then, the wafer is moved to the exposure position, whereby wafer alignment is performed.
The TTL method and the off-axis method have both advantages and disadvantages.
In the TTL method, since the alignment information is detected through a projection lens, the distance (called “base line”) between the exposure position and the position for alignment observation is short. This is advantageous in with respect to a change with time of the base line. In the off-axis method, on the other hand, although there is a disadvantage that the base line is long, since the projection lens does not intervene, there is a small limitation condition to the optical detection system and, generally, it assures higher precision detection as compared with the TTL detection system. In the TTL method, a projection lens which is aberration corrected with respect to the exposure wavelength has to intervene, and it is difficult to design the wavelength of an alignment microscope as broad band because it differs from the exposure wavelength.
As is well known in the art, an alignment observation system using monochromatic light generally has a large process dependency, as compared with an alignment observation system using broad band illumination. Further, as the exposure linewidth is becoming smaller and smaller and in cases where an excimer laser is used as an exposure light source, the difference between the exposure wavelength (248 nm) and the alignment wavelength (e.g., 633 nm of a He—Ne laser wavelength) becomes very large as compared with cases where conventional i-line is used. This results in the production of large chromatic aberration of a projection lens, with respect to the alignment wavelength. Alternatively, provision of a coating film having a high transmissivity both to the exposure wavelength and the alignment wavelength is difficult to achieve. For these reasons, practically, it is very difficult to design an image detection TTL system.
For the reasons described above, improvements of precision with the off-axis alignment method have become more and more important, with further miniaturization of patterns.
FIG. 1A
is a schematic view of a conventional semiconductor exposure apparatus with an off-axis alignment detection system. Since the operation and function of such a semiconductor exposure apparatus are well known in the art, a detailed description will be omitted here. The off-axis alignment detection system
1
is disposed adjacent to a reduction projection lines
2
, and it serves to detect positions of alignment marks provided on scribe lines (not shown) upon a wafer
3
. Denoted at
51
is a mask, and denoted at
52
is a CPU. Denoted at
53
is an illumination system and at
54
is a stage system.
Although, generally, the off-axis method has a smaller process dependency as compared with the TTL method, when total overlay budget is taken into account, the proportion occupied by the process precision is still large. One major factor is a lens effect of a resist covering an alignment mark.
FIG. 1B
is a schematic view for explaining the lens effect of a resist. The upper half illustrates reflected light rays from a mark, and the lower half illustrates an output of a CCD which receives the light rays. While the target of an alignment mark is formed with a predetermined step height (level difference), it is made symmetrically or asymmetrically due to the process or processes. Furthermore, usually in the sectional shape, a resist covers it asymmetrically. A portion of light (A-D) projected on the resist surface is refracted by the inclined resist surface and then is reflected by the surface of a bottom substrate at a predetermined angle. Then, it goes out of the resist surface (A′-D′). At this moment, the light is influenced again by refraction. Due to the double refraction effect, the paths of light rays emitted from left-hand and right-hand edges of the mark are modulated asymmetrically, causing a large factor of process error.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a position detecting method and/or a position detecting system by which a refraction effect at the surface of a resist during observation is removed or reduced, by which a process error attributable to asymmetry of a resist or an alignment pattern is reduced, and by which high precision alignment is assured.
REFERENCES:
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patent: 4901109 (1990-02-01), Mitome et al.
patent: 5309197 (1994-05-01), Mori et al.
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patent: 5793473 (1998-08-01), Koyama et al.
patent: 1-112726 (1989-05-01), None
Kose, et al., Optical Engineering Handbook, Asakura Shoten, pp. 387-396 and 687-691.
Hasegawa Masanobu
Ina Hideki
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Fuller Rodney
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